In a TET system, a power supply is electrically connected to a primary coil that is external to a physical boundary, such as the skin of the human body. A secondary coil is provided on the other side of the boundary, such as internal to the body. With a subcutaneous device, both the primary and secondary coils are generally placed proximate to the outer and inner layers of the skin. Energy is transferred from the primary coil to the secondary coil in the form of an alternating magnetic field. The secondary coil converts the transferred energy in the AC magnetic field to electrical power for the implant device, which acts as a load on the secondary coil.
In a TET system, the primary and secondary coils are placed on separate sides of the boundary or skin. This separation typically results in variations in the relative distance and spatial orientation between the coils. Variations in the spacing can cause changes in the AC magnetic field strength reaching the secondary coil, in turn causing power fluctuations and surges in the implant device. Implant devices, such as those used in medical applications, usually rely upon a microcontroller to perform various functions. These microcontrollers require a consistent, reliable power source. Variations in the supplied power, such as sudden changes in voltage or current levels, may cause the device to perform erratically or fail to function at all. Accordingly, one issue associated with conventional TET systems is that the physical displacement of either the primary or secondary coils from an optimum coupling position may cause an unacceptable effect on the output power supplied to the implanted device.
As an example of an implantable device that may benefit from use of TET is an artificial sphincter, in particular an adjustable gastric band that contains a hollow elastomeric balloon with fixed end points encircling a patient's stomach just inferior to the esophago-gastric junction. These balloons can expand and contract through the introduction of saline solution into the balloon. In generally known adjustable gastric bands, this saline solution must be injected into a subcutaneous port with a syringe needle to reach the port located below the skin surface. The port communicates hydraulically with the band via a catheter. While effective, it is desirable to avoid having to adjust the fluid volume with a syringe needle since an increased risk of infection may result, as well as inconvenience and discomfort to the patient.
To that end, in the above-referenced co-pending applications, an implanted infuser device regulates the flow of saline without requiring injection into the subcutaneous port. This system instead transfers AC magnetic flux energy from an external primary coil to a secondary coil that powers the pump in the implant connected to the gastric band within the abdomen. Although TET is not required for powering the device, the long-term nature of these devices benefits from use of TET, allowing an implanted device of reduced size and complexity. Moreover, these devices may remain unpowered between adjustments, which provides additional advantages such as not requiring a battery.
It is known to surgically implant a medical device such as, for example, a cardiac pacemaker or an adjustable gastric band, under the surface of a patient's skin to achieve a number of beneficial results. In order to actively operate within a patient, these medical implants require a reliable, consistent power source. Currently, medical implants are powered by either non-rechargeable batteries, rechargeable batteries that use a TET system to recharge the batteries, or directly by a TET system. In order to transfer sufficient power to the secondary coil of the TET system to operate an implant, TET systems have typically operated at frequencies from 100 kHz to upwards of 30 MHz. At these higher frequency levels, the alternating electromagnetic field that the primary coil generates couples not only to the secondary coil, but also to any metallic objects near it, including a metallic implant case. This parasitic coupling produces eddy currents in the implant case. These eddy currents reduce the amount of effective power transferred to the secondary coil, thereby increasing the amount of power required from the primary coil to drive the implant. In addition, the eddy currents can cause heating of the metallic case. Heating a metallic implant case by more than 2° Celsius above normal body temperature can have derogatory effects on the implant recipient. The level of eddy currents produced in a metallic object is directly proportional to the alternating magnetic field frequency raised to the second power. Accordingly, the greater the frequency of the energy transfer signal, the greater the eddy currents and energy transfer losses. In addition, operating a TET system at frequencies above 100 kHz requires that the system conform to FCC regulations.
To reduce the problems associated with eddy currents and power transfer losses when using a TET system with an implant, it has traditionally been necessary to enclose the implant in a non-metallic material, such as a ceramic outer casing, or to place the secondary coil outside of the implant outer casing and connect the coil by a pair of leads extending into the casing. Alternatively, TET systems have been used as low energy trickle charge systems that operate continuously to recharge internal implant batteries. Each of these solutions to the eddy current problem, however, is either expensive, cumbersome, or increases the complexity of the implant device. Accordingly, in order to reduce the problem of eddy currents when powering an implant, and minimize the issue of FCC regulations, it is desirable to have an energy transfer system that operates at low frequencies. In particular, it is desirable to provide a high power, low frequency TET system in which the secondary coil may be encased within the implant without significant power losses or development of eddy currents.
Although such TET powering of an implant, such as to recharge batteries, is a generally known procedure, using TET for an artificial sphincter system, such as an adjustable gastric band, presents a number of challenges. Adjustable gastric bands are most beneficial to patients that are morbidly obese. Providing a secure location to subcutaneously attach an implant that presents a reduced incident of discomfort often means that the implant is under a thick layer of skin and adipose tissue. A major challenge in using TET thus is transferring magnetic energy between the primary and secondary coils through this thick layer of dermal tissue, which thus reduces the effective amount of power transferred to the implant.
Consequently, a significant need exists for enhancing TET power transfer through the dermis of a patient and into a hermetically sealed case of an implanted medical device without significant power losses.